For the increase of the recording capacity of optical discs, the wavelengths of light sources are becoming shorter and the numerical apertures (hereinafter, “abbreviated as NA”) of objective lenses are becoming larger. The wavelength of a light source wavelength is 650 nm and the NA of an objective lens is 0.6 for DVD discs, but an optical system whose light source has a wavelength of 405 nm and objective lens has an NA of 0.85 has been proposed for next-generation optical discs.
For the further increase of the recording capacity, a multilayer optical disc in which a multitude of information layers are superimposed at specified intervals in the thickness direction of the optical disc is also being developed. At the time of information reproduction from this multilayer optical disc, there has been a problem of interlayer crosstalk that signals from the other layers leak into signals of the information layer desired to be reproduced. Against this problem, there has been proposed an optical head device for removing reflected lights from the information layers other than the reproduction layer using a confocal optical system.
FIG. 30 is a diagram showing the construction of a conventional optical head device disclosed in patent literature 1. In FIG. 30, identified by 1 is a semiconductor laser, by 2 a collimator lens, by 3 a beam shaping prism, by 4, 5 nonpolarizing beam splitters, by 6 an objective lens and by 7 a multilayer magnetooptical recording medium, wherein information layers are laminated in the multilayer magnetooptical recording medium 7. Further, identified by 8 a half-wave plate, by 9 a polarizing beam splitter, by 10, 11 collective lenses, by 12, 13 pinholes provided at focus positions of the collective lenses 10, 11, by 14, 15 photodetectors, by 16 a cylindrical lens and by 17 a quarter divided photodetector.
In FIG. 30, an information signal of the multilayer magnetooptical recording medium 7 is detected from a difference in light quantities having passed through the pinholes 12, 13, and reflected lights from the layers of the multilayer magnetooptical recording medium other than a reproduction layer are mostly shielded by the pinholes 12, 13. Thus, an optical head device with less interlayer crosstalk can be obtained. In the focus and tracking servo of the information layer of the multilayer magnetooptical recording medium 7, a focus error signal is obtained by a known astigmatic method and a tracking error signal is obtained by a push-pull method or heterodyne method using the cylindrical lens 16 and the quarter divided photodetector 17.
As a tracking method for the multilayer optical disc as described above, there is also proposed the one in which a tracking groove is formed only in one layer out of a plurality of information layers, a tracking error signal is obtained from the tracking groove, a signal is written in a layer different from the one formed with the tracking groove, and then the tracking error signal is obtained based on the written signal and a signal is written in another layer, thereby successively performing tracking between the layers as disclosed in patent literature 2.
Although the reflected lights from the layers other than the reproduction layer are eliminated using the pinholes in patent literature 1, a method obtaining the same effect by reducing the size of a photodetector to about the size of pinholes is disclosed in patent literature 3.
However, in the construction of patent literature 1, the photodetector for servo and the one for information signal detection needed to have the positions thereof individually adjusted and individual displacements thereof needed to be suppressed in response to temperature variation and vibration since being separated from each other. Further, patent literature 2 does not disclose any optical construction combined with the confocal optical system. Furthermore, patent literature 3 does not disclose a specific optical construction although an astigmatism method and a three beam method are disclosed to obtain focus and tracking servo signals.
Patent Literature 1: Japanese Patent No. 2624255
Patent Literature 2: Japanese Patent No. 2835074
Patent Literature 3: Japanese Unexamined Patent Publication No. H08-185640